1. Field of the Invention
The present invention generally relates to the manufacture of very large scale integrated (VLSI) circuits and, more particularly, to methods for wet etch removing an etch layer without leaving an undesired undercut in the process of manufacturing such circuits.
2. Description of the Prior Art
Borderless contact schemes employed in certain integrated circuit (IC) manufacturing processes have adopted an aluminum oxide (Al.sub.2 O.sub.3) conformal film as an etch stop which is subsequently wet etched removed because of dry etch limitations. To properly remove the Al.sub.2 O.sub.3 in known etchants, an undercut equal to the thickness of the Al.sub.2 O.sub.3 results. This poses a problem for reliability and yield in the subsequent metal film deposition. The resulting undercut does not meet desired insulator dimensions for certain IC manufacturing processes. An example of minimum insulator dimensions is a worst case stud to stud spacing no less than 0.4 .mu.m, but this is often not met with a 2500.ANG. undercut in a 2500.ANG. thickness of the Al.sub.2 O.sub.3. The Al.sub.2 O.sub.3 is the film of choice due to its etch stop characteristics. It is therefore required that some method be devised to eliminate this undercut.
U.S. Pat. No. 4,289,574 to Radigan et al. teaches etching aluminum and protecting layers below the aluminum. More specifically, Radigan et al. interpose Al.sub.2 O.sub.3 between a thin aluminum film and a thick aluminum film so that in etching the thick aluminum, the Al.sub.2 O.sub.3 serves as an etch stop. The Al.sub.2 O.sub.3 can be redissolved later, and then the thin aluminum can be wet etched to maintain selectivity to the substrate. The use of multiple layers, as exemplified by Radigan et al., is complicated for the insulator stack case, and finding suitable materials with the right electrical properties is difficult.
U.S. Pat. No. 4,457,820 to Bergeron et al. teaches that in order to control an etch to a layer below, the layer is wet etched to a thickness that stops on the layer below in the thinner regions and then subjected to reactive ion etching (RIE) to maintain image size until all bottom layers are exposed. The undercut (or image size) in this case is controlled by the use of an RIE etch at the right part in the process. The undercut relative to the mask is not important since the mask is removed and the undercut is more easily dealt with.
U.S. Pat. No. 4,838,991 to Cote et al. teaches using an organic parylene spacer to make image sizes smaller; i.e., it is useful per se as an organic spacer and to control the contour of a given first material.